This invention relates to the kiln drying and seasoning of lumber.
Generally in the kiln drying process for lumber, hot air is blown across sawn unseasoned lumber to remove water. The lumber is arranged in stacks with small spacers, commonly referred to as stickers, between each layer so that the hot air is distributed across the faces of the lumber. After passing through the stack of lumber, the hot humid air is either partially vented, or dehumidified, to remove the water vapor before being reheated and recycled through the stacked lumber.
Those skilled in the art will recognize two basic prior art methods of kiln drying, distinguished by the method used for heating and removing of water vapor.
In the first prior art method, standard kiln drying, heat is supplied to the kiln directly, by burning of fossil fuel or indirectly, with steam passing through heat exchangers associated with the kiln. Water vapor is removed from the kiln by venting a portion of the recycled humid air to the atmosphere. Standard kiln drying is energy intensive in that it requires from 2,000 to 5,000 BTU per pound of water evaporated from the wood depending upon kiln construction, wood type, and drying schedule. Additionally, steam spray is required for stress relief of lumber at the end of drying and adds considerably to the energy requirements of the kiln.
In the second prior art method, dehumidification kiln drying, heat is electrically supplied to the kiln initially by a resistance heater and then with a compressor. Water vapor is condensed and removed from the kiln as water. Dehumidification drying has many drawbacks: (1) the drying process is slow taking twice as long as standard kiln drying; (2) the kiln operates at temperatures below 120.degree. F. because of limitations in the dehumidification equipment; (3) drying rates below 10% moisture content are extremely slow and moisture contents of 6% required for furniture-grade lumber cannot normally be achieved; and (4) no capabilities for a humidification period following drying of the lumber required to relieve internal stresses which remain in the lumber are available.
The prior art also includes U.S. Pat. No. 3,965,696--Thomason; showing a crop drying apparatus using a heat pump to produce hot or cold drying air. The reference provides supplementary heat from a solar heat collector used in conjunction with heat obtained from the heat pump. Thomason, however, does no contemplate the disposition of both the condenser and evaporator coils within a single drying building to both produce heat and dehumidify the air passed over the material to be dried, or channels which can be relatively closed off to use the refrigeration system as a heat pump or dehumidifier in various stages.
U.S. Pat. No. 4,205,456--Ayers, et al; shows a heat pump arrangement for drying material in which air is heated as it passes through the material to be dried and is channeled through an evaporator for removing moisture therefrom.
U.S. Pat. No. 3,922,798--McMillan shows a drying apparatus wherein a refrigeration system is wholly contained within the apparatus and the heating for the apparatus is provided via the condenser with moisture removal being provided via the evaporator.
U.S. Pat. No. 3,931,683--Crites et al., shows a drier for material wherein air is passed through both a condenser and the evaporator of a refrigerant system. The condenser is disposed in a material to be dried so as to heat the material as air is passed there through. The heated, moisture-laden air is then passed through the evaporator to remove the moisture therefrom.
U.S. Pat. No. 3,762,065 l issued Oct. 2, 1973 to Wahlgren is cited to show another apparatus for drying materials wherein the condenser of a refrigeration system is used for heating air to be passed through the material to be dried. The evaporator of the refrigeration is used for dehumidifying the air after it has passed through the material.